Modulator for gas chromatography

ABSTRACT

Abstract of the Disclosure 
         
   This invention relates to a modulator for use in gas chromatographic analysis, adapted for alternatively trapping and releasing fractions of solutes in a length of a capillary column within a chromatographic oven, characterized in that it comprises at least one nozzle placed to spray at least one jet in at least one corresponding place along said capillary column length, said nozzle(s) being connected each to a source of liquid C0 2   via a related valve, and means for alternatively opening said valve(s) for a predetermined time, to cause a jet of liquid C0 2   to impinge for said predetermined time on said column place and to leave the oven atmosphere to heat said column place after said predetermined time. The modulator can be used in a conventional GC system or in a two dimensional GC system, for modulating the analytes fed to the second capillary column.

Detailed Description of the Invention FIELD OF THE INVENTION

[0001] This invention relates to a modulator for modulating samplefractions in a capillary column during a gas chromatographic analysis.

[0002] The modulator according to the present invention can be designedfor a traditional gas chromatographic apparatus in order to enhance thesensitivity by narrowing the peaks when placed directly in front of thedetector or to focus the injected analytes when placed directly afterthe injector. However, it can also specially designed for comprehensivetwo dimensional gas chromatography.

STATE OF THE ART

[0003] The comprehensive two dimensional gas chromatography, also calledcomprehensive 2D GC, or GCxGC, is a gas chromatographic technique inwhich the sample is first separated on a conventional normal-borehigh-resolution capillary GC column in the programmed temperature mode.All of the effluent of this first column is then focused in a largenumber of extremely narrow (<100 ins) and adjacent fractions at regular,short intervals and subsequently injected onto a second capillarycolumn, which is short and narrow to allow for very rapid separations.GCxGC can be interpreted as to exist of two GC systems coupled in seriesby means of a so-called modulation system (Fig. 1). The first GC is aconventional capillary GC system, including a conventional injector; thesecond is a fast GC, which is about 50 times faster than the first one.This is accomplished by using a short and narrow-bore column to providevery narrow peaks with peak widths at baseline of 100 - 200 ins. Themodulation system provides the correspondingly narrow injection pulsesin such a way that no sample is lost during the transfer between thechromatographic dimensions. In this way the comprehensive GCxGCtechnique permits to obtain a separation power considerably larger thanthat of conventional capillary gas chromatography, together with animproved sensitivity, a better peak identification and otheradvantageous features.

[0004] As previously said, in order to carry out said GCxGC it isnecessary to operate a so-called modulation system between the first andsecond capillary column in order to retain and focus the narrowfractions of the effluent of said first column and inject the same atintervals onto said second column.

[0005] The most widely used modulators are of the thermal type, 10wherein a thermal action on a column length is used to trap and releasethe fractions to be injected in the second column.

[0006] The known heated modulators use an intermediate, thick filmmodulation capillary to trap (parts of) the eluting analytes from thefirst column by means of phase-ratio focusing. Heat is applied tothermally desorb the analytes from the thick film stationary phase inorder to re-inject the narrow chemical pulses into the second column.Fig. 2 presents this phase-ratio focusing and thermally desorptionprocess in four steps.

[0007] In the first paper describing the comprehensive GCxGC 20technique, by Liu and Phillips [Z.Y. Liu, JB. Phillips, J. Ohrom. Sci.,1991, 29,227-2313 and in the Phillips patent[US 5.196.039] a dual-stagemetal-coated capillary with a thick film of stationary phase, connectedwith the outlet of the first column, but placed outside the oven, wasemployed as a modulation system. Sequentially the two parts of the metalcoated capillary were resistively heated to desorb the analytes trappeddue to the lower temperature of the modulation column and its thickstationary phase film. This system appeared not to be robust enough forlong use and introduced limitations in the lower temperature of the ovenhousing of the two columns (as the minimum temperature of the ovenshould be in this case at least 1000 0 higher than the temperature ofthe modulator which is kept close to the ambient one).

[0008] A more sophisticated heated desorption system was described andmade commercially available by Ledford et al. [J.B. Phillips, R3.Gaines, J. Blornberg, RW.M. van der Wielen, J.M. Dimandja, V. Green, J.Granger, D. Patterson, L. Racovalis, HJ. de Geus, J. de Boer, P.Haglund, J. Lipsky, V. Sinha, E.B. Ledford, J. High Resolut.Ohromatogr., 1999, 22, 3-10], and Phillips and Ledford patent [US6.007.602) mainly consisting of a slotted heater moving along the thickfilm capillary (sweeper) within the gas chromatographic oven.

[0009] However, this system too shows drawbacks, mainly due to themovement of the slotted heater in the close vicinity of the tinycapillary, which causes an easy breakage of the column and a limit ofthe oven maximum temperature.

[0010] In order to render more efficient the fraction trapping andeliminate the necessity of a special thick film capillary length,inserted between the first and second column as well as to remove thelimitations related with the maximum oven temperature, so calledcryogenic or cooled modulators were introduced.

[0011] These modulators, consisting of a cold trap moving sequentiallyforward and backwards along the inlet portion of the second capillarycolumn(the cooling medium sweeps an upstream length of the secondcolumn), cryogenically trapping and focusing (parts of) the analytes asthey elute from the first column on the first section of the secondcolumn itself [R.M. Kinghorn, P.J. Marriott, J. High Resolut.Chromatogr., 1998, 21,620-622]. When the cryogenic system moves awayfrom the zone in which the analytes were trapped, the surroundingGC-oven air quickly heats up the trapped analytes remobilising them forre-injection in the remaining part of the second column. This cryogenictrap, focus and re-injection process is schematically presented in Fig.3.

[0012] The major drawback of this system is the very frequent breakageof the portion of the fused silica capillary column where the cold trapis moving due to ice formation between the cold trap and the column.

[0013] Apart from the mechanical differences between the heated andcooled modulators, there are also some differences in theirapplicability. In the heated modulators a difference in temperature ofat least 100ºC is necessary between the oven and the sweeper, toremobilize the analytes from the thick film capillary that holds theretained fraction. The maximum temperature, to which this capillary canbe heated up, i.e. the maximum allowable temperature of its stationaryphase, determines the maximum operation temperature of the sweeper.

[0014] The maximum temperature of the column oven will be thereforelimited to 100 C below the sweeper temperature and this introducesstrong limitations in the application range covered by such systems.This limitation does not exist with the cooled moving modulator, themaximum operation temperature of the oven can be much higher as it islimited only by the maximum operating temperature of the two separationcolumns themselves.

[0015] The common characteristic of the thermal modulators as they havebeen described, however, is the fact that both techniques use aheating/cooling device that moves across a close distance around afragile fused silica capillary column. Even very accurate (and rathertedious) tuning of these moving devices and their short distance to thecapillaries, frequently leads to breakage of the tiny and fragilecapillaries.

[0016] Ledford [E.B. Ledford, 0. Billesbach, J. High Resol. Ohromatogr.,2000, 23, 202-204] introduced a modification of its heating sweeper, byapplying a cooling jet of 002 on the heating arm. However, this systemand the cryogenic system as previously illustrated show all drawbacks ofthe modulators having movable parts within the oven and moreover thecontinuous jet of 002 tends to create ice formations on the column whichinvolves breaking possibilities and hindering of fraction release.

[0017] Ledford (E.B. Ledford, presented on the 23rd Symposium onCapillary Gas Chromotography, Riva del Garda, Italy, June 2000) recentlyproposed a two-stage liquid nitrogen/heated air jet modulator with nomoving ports. Two cooling and two heating jets spot-cool and -heat avery short section of the second column to trap/focus and re-inject themodulated fractions. The two cooling jets of the two-stage jet modulatoralternately spray liquid nitrogen directly onto the inlet part of thesecond column for trapping/focusing. Two jets with heated gasalternately heat up these spots to remobilize the analytes forre-injection as very narrow pulses.

[0018] The heating jets were necessary, since the temperature of thecooled 20 sections of the second column could reach temperatures as lowas - 190ºC.

[0019] Liquid nitrogen is not easily available at every laboratory andneeds bulky insulation when transported through tubes. Moreover, the useof liquid nitrogen may create problems due to ice formation within theoven and in particular on the jet nozzles which may such hinder or evenstop the release of liquid nitrogen. Moreover, since the hot air jetmust have a temperature at least 100ºC above the oven temperature andvery high air jet temperature cannot be reached for reasons of columnintegrity (maximum temperature of fused silica columns is 350ºC), thislimits the maximum temperature of the oven and the range of applicationscovered by such systems.

OBJECTS OF THE INVENTION

[0020] The object of the present invention is now to provide a modulatorfor GC or GCxGC which optimises the analytes treatment in a conventionalGC system and overcomes the drawbacks of the presently known modulatorsfor GCxGC, in particular with reference to those connected with themobile modulators (sweepers) and with the use of liquid nitrogen and hotair jets in the Ledford modulator with no moving parts.

DESCRIPTION OF THE INVENTION

[0021] The main feature and further features of the modulator accordingto this invention are reported in claim 1 and respectively in thedependent claims.

BRIEF DESCRIPTION OF THE DRAWINGS

[0022] The invention will be more deeply described with reference to theaccompanying drawings, wherein:

[0023]Fig. 1 is a scheme of the GCxGC system.

[0024]Fig. 2 is a scheme of the known heating modulation system(sweeper).

[0025]Fig. 3 is a scheme of the known cryogenic modulation system.

[0026]Fig. 4 is a scheme of a modulator according to the presentinvention.

[0027]Fig. 5 is a detail of the jet configuration of the modulator ofFig. 5.

[0028]Fig. 6 is a chromatogram obtained by means of a GCxGC separationof C₈ through C₁₈ with a modulator according to the present invention.

[0029]Fig. 7 is a chromatogram obtained by means of a GCxGC separationwith a modulator according to the invention and showing the shape of themodulated n-C₁₄ peaks.

[0030]Fig. 8 is a scheme of a modulator according to the presentinvention when applied to a conventional GC system.

[0031]Fig. 9 represents two chromatograms showing the effect of peaksensitivity enhancement.

[0032]Fig. 10a and 10b are diagrammatic representations of analternative embodiment of the jet configuration, respectively in frontview and side view.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0033] Referring to the drawings, Fig. 1 diagrammatically shows thecomponents of a known GCxGC system, preferably housed in a single oven.Fig. 2 shows a scheme of the heating modulation process, in which thefraction eluting from the first column is trapped at the upstream end ofthe thick film of the modulation capillary (phase trapping) (step 1);when the heating sweeper comes in correspondence of this capillaryupstream end, the heat effect releases the fraction (step 2) andtransports the same along the thick film capillary, while a furtherfraction is trapped at the capillary upstream end (step 3).

[0034] When the sweeper reaches the second column, the first fraction isreleased on the same, while the further fraction is still trapped at themodulation capillary upstream end (step 4).

[0035]Fig. 3 schematically shows the cryogenic modulation process,wherein a coolingmedium cools an upstream length of the second column.In correspondence of the cooling medium the fraction is trapped bythermal action and then released when the cooling medium is removed.

[0036]Fig. 4 is a scheme of a GCxGC system with a modulator according tothis invention. The system comprises, in a GC oven 8, an injector 1, afirst column 2 and a second column 6 which are connected at 3 accordingto a well known technique. The second column 6 ends in a detector 7.

[0037] On an upstream length 9 of the second column 6 two jets 4A and 48operate alternatively and at a suitable frequence, which are fed,through corresponding valves 5A and 58, by a source of liquid CO₂ 10 sothat two parts of the capillary length 9 are directly cooled alternatingin order to trap and focus the fraction, whereafter they are remobilizedby the heat of the surrounding oven air. The opening time of each valveis preferably the same for all valves and half the cycle time, while theopening and closure of the valves are carried out in sequence to cover acycle time in the order of 0.1 to 30 seconds. It is to be noted that theopening time of said valves could also be different and that thisopening time may vary from about 0.1 to about 30 seconds.

[0038] The CO₂ jets in Fig. 5 consist of two electrical-driven two-wayvalves 5A, 5Bthat open and close the liquid-CO₂ line alternating throughtwo pieces of 40 mm long, 0.8 mm ID capillaries 11A/11B, coupled to thenozzles (12A, 12B), 50 mm long 0.5 mm ID capillaries. In order to forceas much CO₂ from the outlet of the jets to touch the column, the outletshave been modified to form a slit, 0.04 mm wide and 3 mm long, inparallel above the capillary. To prevent ice formation onto the outsideof the jets at oven temperatures below about 100⁰C, they have beeninserted in a 12 mm diameter brass socket to increase the heat capacity.

[0039] An alternative embodiment of the jet configuration is shown infigs 10a, 10b and 11, wherein, instead of the slit, the outlet isconstructed by inserting a series of seven capillaries in a row betweenthe same brass half blocks. More detailedly, as shown in figs 10a and10b, each brass block 20 houses a stainless steel capillary 21, forinstance having 1/16" OD and 0.7 mm ID, said capillary 21 beingconnected through a related valve 15, to the C02 source 10. Within theend of capillary 21 are inserted for instance seven capillaries 22placed according to what is shown in fig. 11 and fixed preferably by aceramic glue or soldering 23, which is able to withstand temperatures ofup to 400⁰C. In the shown example the capillaries have the followingdimensions: length 35 mm. OD 0.23 mm, ID 0.11 mm and their free portionsare aligned so to run in parallel with the secondary GC column 9 so thatan optimum heat exchange is enabled by generating a "curtain" ofexpanding CO₂.

[0040] The axes of the outlet openings of the capillaries 22 are placed0.4 mm apart, so that the total length of the nozzle again is 3 mm. Ofcourse, the above stated number and dimensions of capillaries can bechanged at will.

[0041] The above stated construction allows to decrease the consumptionof CO₂ and optimize the effectiveness of the throttling process at thenozzle outlet of the cryogenic jets.

[0042] As the liquid C0₂ expands at the outlet of the nozzles, thethrottling process cools the departing gas through the Joule-Thompsoneffect. Since this gas is sprayed directly onto the second column length9 at the prevailing flow, the column quickly cools down to about 100ºCbelow the oven temperature. Closing the valve will immediately stop thecooling process and the surrounding air from the stirred oven will heatup the short cooled section of capillary (about 10 mm) momentarily tooven temperature. The time required to heat the capillary column fromcryogenic to oven temperature is only 13 ms for a normal 100 µm column(15 µm polyimide and 80 µm fused silica walls).

[0043] The length 9 of the second column in which the modulation takesplace, is stretched and secured between two Valco unions 13 mounted on abracket 14. The stretching is necessary in order to avoid vibration ofthe column caused by the rather intense flow of cold CO₂ that is sprayedonto the column. The unions are mounted onto two bands of 1 mm thick,resilient steel in order to compensate for the difference in thermalexpansion of the steel bracket and the fused silica column.

[0044] A simple timing device that generates the 24 DC voltages forvalve switching controls the modulation process. Modulation timesshorter than 0.3 seconds can be established.

[0045] In order to test the performance of the modulator according to 10the invention, a gas chromatograph was used with a split/splitlessinjector and a Flame Ionisation Detector capable to produce a digitalsignal sampled at 200 Hz rate. The first dimension column 30 m x 0.32 mmID was coated with methylsilicon polymer, 0.25 microns film thickness.It was coupled through a press-fit connector to the second column 1.5 mx 0.10 mm ID, which was coated with 0.1 µm BPX5O (SGE International,Ringwood, Australia). The flow was set to 1.0 mL/min through a columnhead pressure of 170 kPa helium. The columns were temperature programmedfrom 50⁰C, 4 mm isothermal, 2⁰C/min to 300⁰C.

[0046] The main functions of the modulator are twofold: focusing smallfractions from the effluents of the first column into narrow pulses andre-injection of these pulses into the remaining part of the secondcolumn. To judge the performance of the modulator, it is sufficient tomeasure or calculate the bandwidth of the injected pulses. To judge theperformance of the dual jet modulator, a series of n-alkanes (C₈ throughC₁₈, see Fig. 6) was separated. From calculations of the peaks modulatedfrom n-C14 (see Fig. 7), the peak widths are σ = 30 ms, which is betterthan second dimension peaks previously reported in the literature forknown modulation systems (sweeper and cryomodulators). The injectionbandwidth appeared to be σ< 10 ms, which is also better than theinjection bandwidths of the known sweeper and cryomodulators.

[0047] According to what stated above, the jet modulator of thisinvention is s very simple in construction and easy to install andmaintain. Its control is performed by simply switching one, two or morevalves, so that no movable part are foreseen within the oven, thuspreventing any column breakage due to movement of the previously knownmovable modulators.

[0048] Moreover, it has been ascertained that the ability of themodulator according to the invention to focus the trapped firstdimension fractions into narrow pulses is superior to that of themodulators known, tested and described in the prior art.

[0049] It is to be finally noted that the present modulator, whendesigned with one liquid CO₂ jet only, can act as an injection focusingdevice and/or as a peak narrowing and then a detector sensitivityenhancing device in a conventional one-dimensional GC system. Thisconfiguration is depicted in fig. 8, where a capillary column 2 isconventionally housed in an oven and connected with an injector 2 and adetector 3. A jet of liquid C0₂ issued by a source outside the oven andcontrolled by a valve, placed outside the oven, can be foreseen toimpinge on a column portion respectively directly after the injector(position A) and/or immediately before the injector (position B).

[0050] When in position A, the CO₂ jet allows to focus the injectedanalytes, while when in position 8 the jet enhances the sensitivity ofthe detector by narrowing the peaks.

[0051] This is confirmed by the chromatograms of fig. 9, comparing thedetector response under the same conditions respectively withoutsensitivity enhancement (C0₂ jets in position A and B not operative) andwith sensitivity enhancement (CO₂ jet in position A not operative andC0₂ jet in position B operative). A series of low concentrationimpurities in a main component are shown in the chromatograms of fig. 9,wherein the upper chromatogram shows the main peak together with aseries of low concentration impurities in the conventional way, wherethe lower chromatogram shows how these impurities are collected by meansby the single liquid CO₂ jet in position B (at the time of valve on) andreleased as a series of sharp peaks (at the time of valve off) atincreased peak intensities.

What is Claimed is:
 1. A modulator for use in gas chromatographicanalysis, adapted for alternatively trapping and releasing fractions ofsolutes in a length of a capillary column within a chromatographic oven,characterized in that it comprises at least one nozzle placed to sprayat least one jet in at least one corresponding place along saidcapillary column length, said nozzle(s) being connected each to a sourceof liquid C0₂ via a related valve, and means for alternatively openingsaid valve(s) for a predetermined time, to cause a jet of liquid C0₂ toimpinge for said predetermined time on said column place and to leavethe oven atmosphere to heat said column place after said predeterminedtime.
 2. A modulator according to claim 1, characterized in that saidvalve(s) is (are) alternatively opened for a predetermined time within agiven cycle time and in that said column place is heated by the ovenatmosphere during the remaining cycle time.
 3. A modulator according toclaim 2, for trapping and releasing in sequence fractions of solutes,characterized in that it comprises at least two nozzles placed to sprayliquid C0₂ jets in at least two corresponding separated places alongsaid capillary column length, and means for alternatively opening saidvalves each for a predetermined time in sequence within a given cycletime, to cause each jet of liquid C0₂ to impinge for said predeterminedtime on the corresponding column place and to leave the oven atmosphereto heat said column place during the remaining cycle time.
 4. Amodulator according to claim 3, wherein said predetermined time is thesame for all valves.
 5. A modulator according to claim 3, wherein saidpredetermined time is different for at least two of said valves.
 6. Amodulator according to claim 4 or 5, wherein said predetermined time isranging from about 0.1 seconds to about 30 seconds. 7.A modulatoraccording to one of claims 4, 5 or 6, wherein said cycle time is the sumof the predetermined times of all valves. 8.A modulator according to oneof the claims 2 to 7, wherein said cycle time is ranging from about 0.1seconds to about 30 seconds. 9.A modulator according to one of thepreceding claims, wherein each said nozzles has an opening in the formof a slit parallel to said capillary length. 10.A modulator according toclaim 9, wherein said slit is about 0,04 mm wide and about 3 mm long.11.A modulator according to one of claims 1 to 8, wherein each saidnozzle is formed by a set of capillaries aligned in parallel to saidcapillary column length. 12.A modulator according to claim 11, whereinthe upstream end of said capillaries open in a common C0₂ feeding duct,to which the capillaries are glued or soldered. 13.A modulator accordingto claim 12, wherein said capillaries each have an inner diameter of theorder of 0.11 mm and each set forms a curtain having a length of about 3mm.
 14. A modulator according to one of the preceding claims, whereinsaid nozzle(s) is (are) inserted in a metal socket. 15.A modulatoraccording to claim 14, wherein said socket is in the form of a brasstube. 16.A modulator according to one of the preceding claims, whereinsaid column length is mounted in stretched conditions. 17.Use of amodulator according to one of the claims 1 to 15 for modulating thesolute fractions issued by a first chromatographic column and to be fedto a second chromatographic column in a comprehensive two dimensionalgas chromatographic system. 18.Use of a modulator according to one ofthe claims 1 or 2 and 8 to 16 for modulating the injected fractionsimmediately downstream the injector in a gas chromatographic system.19.Use of a modulator according to one of the claims 1 or 2 and 8 to 16,for modulating the eluting fractions from a gas chromatographic columnimmediately upstream the detector of a gas chromatographic system.